This happened on a small tanker fitted with a 9 cylinder MaK diesel engine 450mm bore producing 5,520 kW driving through to a cpp.
Due to a knocking heard in the engine, No 7 unit was overhauled. On inspection, the liner was found to be badly worn and so the unit was fitted with the spare liner, piston and con rod. .
A week later, whilst in port units 1,2 and 4 liners were inspected by taking the exhaust valves out and checking the wear ridge. Excessive wear was found on No 2 unit.
The following day The second engineer was on watch in the engine-room performing routine maintenance. At about 1112, he heard a thump from the main engine and observed white smoke and oil vapour coming from the explosion relief doors on the starboard side of the main engine. Realizing that a crankcase explosion had occurred, he manually shut down the engine and evacuated the engine room. There was no fire and none was injured.
An initial investigation revealed that the No. 8 liner, piston, gudgeon pin and connecting rod had sustained damage and would have to be replaced. As the spares had been utilized for repairs to the No. 7 unit, no permanent repairs could be carried out and the vessel was towed to port.
On inspection it was found that several of the liners including that of No 8 were excessively worn and past the limit of wear recommended by the manufacturer. (recommended max dia 451mm). The ring grooves were badly worn which had led to excessive oil consumption which in turn carbonises in the grooves, jamming the rings and leading to excessive wear and ring breakage. The top ring on No 8 piston had broken into several pieces and the oil scraper ring was broken in two. There were signs of heavy scuffing and scoring in the liner, piston crown and skirt.
The liner wear rate for the liner for the 4000 hours previous to the explosion was calculated to be 25 times the manufacturers recommended maximum rate. (0.01 - 0. 015mm/1000 hrs) The liner had also exceeded the ovality limit of 0.4mm.
The scoring and the material deposited on the No. 8 liner, and the scuffing of the piston crown depict conditions that would be consistent with a breakdown of the cylinder liner lubrication film. This breakdown would have resulted in metal-to-metal contact between the piston rings and the liner with consequential hot spots on the liner. Further, the combustion gas blow-by can also ignite hydrocarbon vapours. Thus, these hot spots and/or combustion gas blow-by would likely cause ignition of the lubricating oil vapour/mist, resulting in the crankcase explosion.
Prompt action by the second engineer in shutting down the main engine likely averted a more serious secondary explosion.